1. Field of the Invention
The invention relates to temperature compensation of a thermal mass flow meter.
2. Description of the Prior Art
Thermal anemometers are used to measure the mass flow rate of a fluid, especially a gas at changing temperature. It is known to provide an anemometer having two temperature-dependent electric resistors, usually in the form of thin-film resistors or wires, connected together with two temperature-independent electric resistors, one of which is variable, in a bridge. An electric regulating circuit controls the operational voltage of the bridge in such a manner that one of the temperature-dependent resistors is electrically heated to a constant temperature that is in excess of the temperature of the unheated temperature-dependent resistor and of the temperature of the medium to be flow measured. This makes it possible to measure the mass flow of fluids fairly reliably even if the fluid temperature is changing. The change of the bridge current is a measure of the mass flow of the flowing fluid.
A thermal anemometer for measuring the mass flow of air with improved measuring accuracy is described in DE-OS No. 28 43 019 and in the publication "DISA-Information 22, 1977, pp. 5 to 14". The bridge contains, in addition to the two temperature-dependent electric resistors, three temperature-independent electric resistors, two of which are variable resistors. An excess temperature of the heated resistor in relation to the unheated temperature-dependent resistor is set with the one variable resistor R.sub.UT at a typical medium temperature. A difference in the temperature coefficients of the electric resistance of the two temperature-dependent electric resistors and a positive temperature dependency of the coefficient of heat transfer from the heated resistor to the fluid is compensated by using the other variable resistor R.sub.TK, which can also be a resistance network. The temperature dependency of the excess temperature in relation to the medium temperature is varied with this electric resistor. The bridge current provides a measuring signal indicative of mass flow rate.
However, this known arrangement for temperature compensation has disadvantages. There is an unavoidable spread (difference) e.g. in temperature characteristics among various units of the temperaturedependent electric resistors used as sensors and in thermal conductivity of their respective holders. Therefore, the resistor R.sub.TK must be determined individually for each measuring device by measuring the temperature characteristic of the measuring signal in order to obtain exact temperature compensation. However, a change of resistor R.sub.TK necessarily changes the set excess temperature at a typical medium temperature, which must then be reset by changing the resistance value of R.sub.UT. This balancing process must be performed several times, if necessary, and is quite time-consuming.